Most vehicles in operation today (and many other devices) are powered by internal combustion (IC) engines. An internal combustion engine typically has a reciprocating piston which oscillates within a working chamber or cylinder. Combustion occurs within the cylinder and the resulting torque is transferred by the piston through a connecting rod to a crankshaft. For a four-stroke engine, air, and in some cases fuel, is inducted to the cylinder through an intake valve and exhaust combustion gases are expelled through an exhaust valve. In typical engine operation the cylinder conditions vary in a cyclic manner, encountering in order an intake, compression, power, and exhaust stroke in a repeating pattern. Each repeating pattern may be referred to as a working cycle of the cylinder.
Internal combustion engines typically have a plurality of cylinders or other working chambers in which an air-fuel mixture is combusted. The working cycles associated with the various engine cylinders are temporally interleaved, so that the power stroke associated with the various cylinders is approximately equally spaced delivering the smoothest engine operation. Combustion occurring in the power stroke generates the desired torque as well as various exhaust gases. Some of these gases, such as carbon monoxide, hydrocarbons and nitrogen oxides, are pollutants that are harmful to human health.
Governments have implemented regulations to reduce the emission of such pollutants. As a result, modern vehicles include catalytic converters or some other emission control device, which help to remove the pollutants from the exhaust of the engine. One problem, however, is that that catalytic converters do not operate effectively at low temperatures, which are typically encountered when starting an engine. That is, hot exhaust from the fired cylinders must pass through the catalytic converter for a short period of time before it becomes warm enough to effectively filter pollutants. As a result, during the initial engine startup period, pollutants may pass through the catalytic converter without being captured.
There have been various efforts to more rapidly heat the catalytic converter during this startup period to limit the emission of harmful pollutants. One approach, which involves a secondary air injection system, is illustrated in FIG. 1. FIG. 1 is a representative block diagram including an engine 112 with two banks of cylinders 102a/102b, two upstream catalytic converters 106a/106b, a downstream catalytic converter 108 and an air pump 110. Each bank of cylinders is connected to an associated catalytic converter 106a/106b, each of which then are separately connected to a single downstream catalytic converter 108 via a Y pipe 111. During the engine startup period, a rich air-fuel mixture is delivered to all of the cylinders. The cylinders are fired and the resulting exhaust is passed from the cylinders to the upstream catalytic converters 106a/106b. Because of the rich air-fuel mixture, the exhaust contains unburned hydrocarbons, which enter the catalytic converters 106a/106b. The air pump 111 injects additional air into the catalytic converters. The air exothermically reacts with the hydrocarbons. The reaction more quickly heats the catalytic converter to a desired operating temperature.
Effective operation of a catalytic convert occurs over an operational temperature range; for example 400° to 600° F. Excessively high temperatures may damage the catalytic converter. Hot exhaust gases from an engine operating at high loads can exceed an operational temperature range of a catalytic converter and damage the catalytic converter.
Accordingly, there have been various modifications to avoid catalytic converter damage due to high exhaust gas temperature. Among those modifications are changes in the amount of fuel injected and limitations to the engine operating range.